Combine 3 egg yolks, 1 ½ cups water, ½ cup lemon juice, 1 ¼ cups sugar, and 1/3 cup cornstarch in the top of a double boiler. Mix well. Bring water to a boil in double boiler; reduce heat, and cook egg mixture over hot water, stirring constantly, until mixture thickens and comes to a boil. Boil 1 minute, stirring constantly. Remove from heat. Stir in 3 tablespoons butter, 2 tablespoons white vinegar, and 1 ½ teaspoons lemon extract, making sure that all ingredients are incorporated into the pie filling. Pour immediately into baked 9-inch pie crust, and smooth out the top with the back of a spoon. Make meringue for topping and spread meringue over warm filling, sealing to edge of pie crust. Bake at 425 degrees (F) for 5 to 8 minutes, or until lightly browned. Cool before serving. This is a light-tasting pie that is sweet, tart, tangy, and delicious! I hope you love it! Love, Betty